MySQL 中的精确小数除法

Question

如何正确划分MySQL中的两个DECIMAL值并得到精确到列类型中定义的位数的结果？

示例：

select cast(1/2 as decimal(4,4)),
cast(1 as decimal(4,4))/2,
cast(1 as decimal(4,4))/cast(2 as decimal(4,4));

结果：

'0.5000', '0.49995000', '1.00000000'

为什么第二个结果不准确？为什么第三个结果不是'0.5000'？

注意：我不能只使用第一种形式，我需要对存储为小数的列进行计算。

Answer 1

问题是 DECIMAL 数据类型声明需要 2 个参数，第一个是总位数，包括小数部分。

根据文档 (http://dev.mysql.com/doc/refman/5.7/en/precision-math-decimal-characteristics.html)：

The declaration syntax for a DECIMAL column is DECIMAL(M,D). The ranges of values for the arguments in MySQL 5.1 are as follows:

M is the maximum number of digits (the precision). It has a range of 1 to 65. (Older versions of MySQL permitted a range of 1 to 254.)

D is the number of digits to the right of the decimal point (the scale). It has a range of 0 to 30 and must be no larger than M.

因此，如果您使用 5 位而不是 4 位，一切正常：

select cast(1/2 as decimal(5,4)),
cast(1 as decimal(5,4))/2,
cast(1 as decimal(5,4))/cast(2 as decimal(5,4)),
cast(cast(1 as decimal(5,4))/cast(2 as decimal(5,4)) as decimal(5,4));

结果

0.5000, 0.50000000, 0.50000000, 0.5000

MySQL 中的精确小数除法

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